Pitch-based carbon fibers have a higher elastic modulus than PAN (polyacrylonitrile)-based carbon fibers, and therefore, are used in making composite materials based on plastics, metals, carbon, ceramics and so forth. Furthermore, it is expected that they will find a wider field for applications, for example as lightweight structural materials or heat-resistant materials for use in aircraft, spacecraft and equipment, automobiles and the like.
A method of producing pitch-based high-modulus carbon fibers is disclosed, for example, in Japanese Kokai Tokkyo Koho No. 120112/1988. The method disclosed in the above-cited publication is characterized by reducing, in rendering spun pitch fibers infusible (stabilization), the thickness of the infusible layer to thereby suppress the formation, in the product carbon fibers, of a surface layer with a low Young's modulus. In this method, however, the thickness of the infusible layer may occasionally become insufficient, so that fusion may occur in the step of carbonization.
Another method proposed for increasing the modulus of carbon fibers comprises carrying out the heat treatment for carbonization, which follows treatment for rendering pitch fibers infusible, at an increased temperature. However, this method is disadvantageous in that the yield is low and in that the physical properties of the product carbon fibers, such as tensile strength and elongation percentage, tend to decrease.